Gas-liquid contactors are widely used to remove substances such as gases and particulate matter from combustion or flue gases produced by utility and industrial plants. Often of particular concern are sulfur dioxide (SO.sub.2) and other acidic gases produced by the combustion of fossil fuels and various industrial operations. Such gases are known to be hazardous to the environment, such that their emission into the atmosphere is closely regulated by clean air statutes. The method by which such gases are removed with a spray tower or other type of gas-liquid contactor is known as wet flue gas desulfurization (FGD).
The cleansing action produced by a gas-liquid contactor is generally derived from the passage of gas upwardly through a tower countercurrently to a descending liquid which cleans the air. Wet flue gas desulfurization processes typically involve the use of calcium-based slurries or sodium-based or ammonia-based solutions. As used herein, a slurry is a mixture of solids and liquid in which the solids content can be any desired level, including the extreme condition in which the slurry is termed a moist solid. Examples of calcium-based slurries are limestone (calcium carbonate; CaCO.sub.3) slurries and hydrated lime (calcium hydroxide; Ca(OH).sub.2) slurries formed by action of water on lime (calcium oxide; CaO). Such slurries react with the acidic gases to form precipitates which can be collected for disposal or recycling. Intimate contact between the alkaline slurry and acidic gases which are present in the flue gases, such as sulfur dioxide, hydrogen chloride (HCl) and hydrogen fluoride (HF), result in the absorption of the gases by the slurry. Thereafter, the slurry is accumulated in a tank.
A known type of gas-liquid contactor is a spray tower 10 shown in cross-section in FIG. 1. The spray tower 10 generally is an upright structure composed of a tower 14 equipped with an inlet duct 12 through which combustion gases enter the tower 14. Above the inlet duct 12 is a lower bank of spray headers 16 which introduce a spray 20 of an alkaline slurry into the tower 14. A second, upper bank of spray headers 18 is typically provided above the lower bank of spray headers 16, with additional banks of spray headers being used as required for a given application. One or more pumps 26 are required to recycle the alkaline slurry by pumping the slurry from a tank 30 to the banks of spray headers 16 and 18. Each bank of spray headers 16 and 18 may be individually equipped with a pump 26 for the purpose of promoting the flexibility of the pumping and spraying operation to accommodate varying demands by the scrubbing operation.
Intimate contact between the alkaline slurry spray 20 and the flue gases rising through the tower 14 results in a cleansing action, by which the slurry and the entrapped or reacted gases are collected at the bottom of the tower 14 in the tank 30. The cleansed gases which continue to rise through the tower 14 then typically pass through a mist eliminator 22, and thereafter are either heated or passed directly to the atmosphere through a chimney 24.
Due to the large quantity of slurry which must be pumped to scrub the flue gases, a significant cost in the construction, operation and maintenance of gas-liquid contactors is attributable to the pumps 26. Yet, the pumps 26 also constitute a significant limitation to the scrubbing operation, in that the quantity of slurry pumped by the pumps 26 cannot be readily adjusted to accommodate changes in the scrubbing operation, such as the amount of flue gas which must be scrubbed or the amount of contaminants present in the flue gases.
Another limitation of prior art gas-liquid contactors is the relatively low solids content permitted when using a slurry as the cleaning liquid. Typically, the solids content of such slurries must be limited to about ten to about fifteen weight percent. However, higher concentrations would allow the use of a smaller tank 30, since its size is generally dictated by, among other things, the residence time for crystallization of solids within the slurry. Higher solids contents would also eliminate the requirement for primary dewatering devices such as thickeners or hydrocyclones, which are well known devices employed in the art to remove solids and/or byproducts from a slurry. However, high solids contents significantly increase erosion within the tower 14, tank 30, fluid conduit, spray headers 16 and 18 and pump 26, while also increasing the power required to pump the slurry due to the higher specific gravity of the slurry.
Finally, it would be advantageous to maximize the flue gas velocity within the tower 14 from the standpoint of improving contact between the slurry and the flue gases, so as to enable a reduced slurry flow to the tower 14. Higher flue gas velocities would also allow for the use of a tower 14 having a smaller cross-sectional area, such that the cost of constructing the spray tower 10 is reduced. However, conventionally-accepted design practices typically limit the flue gas velocity within the tower 14 to about ten feet per second (about three meters per second) in order to assure the proper operation of the mist eliminator 22. Higher flue gas velocities within the tower 14 tend to increase the gas pressure drop within the tower 14, and therefore increase the likelihood of liquid particles being carried to and flooding the mist eliminator 22.
Those skilled in the art will appreciate that, in view of the considerations noted above, it would be desirable if a flue gas scrubbing apparatus were available which overcame the above-noted disadvantages associated with the use of slurry pumps, yet could employ slurries having higher solids concentrations and higher flue gas velocities.